<!DOCTYPE html>
<html lang="en">

<head>
  <meta charset="UTF-8">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <title>Document</title>
  <style>
    .btn-group button.current {
      background-color: orange;
      color: #fff
    }
  </style>
</head>

<body>
  <div class="J_calculator">
    <div class="result">0</div>
    <div class="input-group">
      <input type="text" value="0" class="f-input" />
      <input type="text" value="0" class="s-input" />
    </div>
    <div class="btn-group">
      <button data-field="plus" class="current">+</button>
      <button data-field="minus">-</button>
      <button data-field="mul">*</button>
      <button data-field="div">/</button>
    </div>
  </div>
  <script src="./demo.js"></script>
</body>

</html>